1. Field of the Invention
The present invention relates to solid-state imaging devices and more particularly, to a thinned charge-coupled imaging device in which both the front and back surfaces can be used simultaneously for image sensing.
2. Description of the Prior Art
Charge-coupled devices are typically made of silicon and are used as solid-state imagers by taking advantage of the properties of a silicon crystal lattice. In the crystalline form, each atom of silicon is covalently bonded to its neighbor. Energy greater than the energy gap of about 1.1 V is required to break a bond and create an electron hole pair. Incident electromagnetic radiation in the form of photons of wavelength shorter than 1 um can break the bonds and generate electron hole pairs.
The wavelength of incoming light and the photon absorption depth are directly related, the shorter the wavelength, the shorter the penetration depth into the silicon. Silicon becomes transparent at a wavelength of approximately 1100 nm and is essentially opaque to light at wavelengths shorter than 400 nm. High energy particles, X-rays and cosmic rays can break many thousands of bonds; therefore, excessive exposure can cause damage to the crystal lattice. Bonds can also be broken by thermal agitation. At room temperature, approximately 50 bonds per second per um.sup.3 are broken and recombined on a continuous basis. The rate of electron hole pair generation due to thermal energy is highly temperature-dependent and can be reduced arbitrarily through cooling.
In order to measure the electronic charge produced by incident photons, it was required to provide a means for collecting this charge. Thus, the potential well concept was developed, wherein a thin layer of silicon dioxide is grown on a section of silicon, and a conductive gate structure is applied over the oxide. The gate structure is formed in an array of columns and rows, thus making it possible by applying a positive electrical potential to various gate elements to create depletion regions where free electrons generated by the incoming photons can be stored.
By controlling the electrical potential applied to adjacent gates, the depletion region, or well, containing the free electrons can be caused to migrate along a column or row, so that the signal may eventually be output at the edge of the array.
Typically, the gate structure is arranged with multiple phases, particularly three phases, so that the potential wells may be easily migrated through the silicon to an output device.
In reality, the wells and the migration of the wells is not carried out along the surface of the silicon-silicon dioxide interface, but takes place in a buried channel below the surface. The buried channel is free of interference from interface states and thus assures effective charge transfer from well to well. The operation of a charge-coupled device is somewhat analogous to that of a bucket brigade circuit commonly used to delay electrical signals.
Because the charge from the wells located far from an output amplifier must undergo hundreds of transfers, the charge transfer efficiency of a charge-coupled device is most important, as is the quantum efficiency and the spectral response. These considerations are particularly important when extremely low light levels are to be sensed.
Light normally enters the charge-coupled device by passing through the gates in the silicon dioxide layer. The gates are usually made of very thin polysilicon, which is reasonably transparent to long wavelengths but becomes opaque at wavelengths shorter than 400 nm. Thus, at short wavelengths, the gate structure attenuates incoming light.
In an effort to overcome this difficulty, it has become the practice to uniformly thin a charge-coupled device to a thickness of approximately 10 um. Using a thinned charge-coupled device, it then becomes possible to focus an image on the backside of the charge-coupled device, where there is no gate structure that will attenuate the incoming light. Thinned charge-coupled devices exhibit high sensitivity to light from the soft X-ray to the near-infrared region of the spectrum.
FIG. 1A illustrates schematically a cross-section of a typical thick-bodied charge-coupled device. The device includes a silicon body 2, a silicon dioxide layer 4 and a gate array 6 formed on the silicon dioxide layer. Incoming light is illustrated by arrows 8 as illuminating a front side of the silicon 2. FIG. 1B illustrates a cross-section of a thinned charge-coupled device with light illuminating a backside. The thinned charge-coupled device, having a thickness of approximately 10 um, has improved quantum efficiency and UV spectral response.
In advanced optical systems, there are instances when it is required that two images originating from separate optical sources be precisely registered onto the surface of an image sensor. Using conventional charge-coupled solid-state image sensors, the requirements are achieved using beam splitters and associated optical elements to properly combine the two optical images into a single path for projection onto a surface of an image sensor. Use of these optical elements is rather conventional; however, it requires precise alignment of the components and inherently introduces unavoidable optical losses as the light passes through the optical elements.
There are numerous applications for solid-state image sensors, wherein the light to be sensed is very dim or does not have a sharp contrast with background light; and in such cases, optical losses are critical. One example of such an application is in guidance systems for space vehicles which utilize terrestrial positioning.
In such applications an image sensor, and in particular a large charge-coupled imaging device array, is used to detect and precisely determine the position of guide stars relative to the vehicle's orientation. The field of possible guide stars is vast, and it would be impossible for a charge-coupled imaging device to be large enough to cover the entire field. Thus, it has become the practice to position the imaging device in an X-Y plane in order to cover the entire field of guide stars. When a positionable imaging device is utilized, it is necessary that the position of the imaging device relative to vehicle axis be precisely tracked.
Conventional ways for determining the imaging device's position rely on either precision mechanical encoders or on optical fiducial marks imaged onto the imaging device array via beam splitters. Given the required precision, the former mechanical method leads to complex solutions, and the latter optical method is wasteful of guide star photons, due to the beam splitter and other image-combining optics.
Referring to FIG. 2, there is shown schematically a prior art system used to orient a space vehicle relative to a guide star field. The heart of the system comprises a charge-coupled imaging device 10 of the standard single-sided construction. Said device is mounted to a movable stage 12 constructed to locate the device 10 in desired positions in an X-Y plane under the control of a stage control 14. A module 16 is provided for processing electrical signals coming from the imaging device.
Terrestrial light or photons from guide stars are collected by a telescope and are directed towards a mirror 18 to be focused onto an aspherical collimator 20 which, in conjunction with an imaging lens 22, directs the guide star photons onto a surface of the imaging device 10. As can be seen from FIG. 2, the photons pass through a beam combiner 24. An illumination driver 26 controls a pinhole grid plate 28 which establishes an array of fiducial reference points which are fixed in position relative to the main telescope axis. The light from the pinhole grid plate 28 is focused by a light grid lens 30 onto the beam combiner 24, where the light images from the grid 28 are combined with the photons from the guide stars and are focused onto the imaging device 10. Based on the location of the fiducial reference points, the signal process electronics module 16 may determine precisely the position of the imaging device in the X-Y plane.
With this configuration, the beam splitter and combiner 24 has the unavoidable effect of diverting a portion of the guide star light away from the imaging device, with a resultant loss of photons and a reduction in the signal-to-noise ratio. Any fluctuations in the position of the pinhole grid plate, light grid lens, beam combiner or the imaging lens results in apparent guide star position error.